The general goal of these studies is to further our understanding of the structure, function, assembly and turnover of two protein complexes associated with the electron transport chain of chloroplast membranes: photosystem II (PSII) and the chlorophyl1 a/b light harvesting complex (chl a/b LHC). Previous studies have revealed that these complexes undergo changes in composition and/or spatial organization in response to various environmental stimuli and stresses. In turn, these changes seem to be involved in the regulation of functional activities and in the reduction of damage to the electron transport chain under conditions of stress. Of particular interest to us are the quinone-binding protein QB of PSII, which turns over 50x faster than other components in the complex, and the unique fatty acid trans hexadecenoic acid that seems to stabilize chl a/b LHC oligomers. The major specific aims are: 1) To characterize differences in composition and structure of PSIIalpha (PSII of stacked membrane regions) and PSIIbeta (PSII of unstacked membrane regions). 2) To determine the spatial organization of three proteins associated with water-splitting in PSII. 3) To characterize the association of different forms of the QB protein with PSIIalpha and PSIIbeta during and following photoinhibition, and during recovery from photoinhibition. 4) To identify and characterize ther components that form the PSII-bound form of chl a/b LHC. 5) To study how phosphatidylglycerol molecules containing Delta3 trans hexadecanoic acid interact with and stabilize chl a/b LHC particles in bilayer membranes. The studies will involve the use of normal, mutant and lipid-enriched chloroplast membranes isolated from barley, pea, Arabidopsis and Chlamydomonas in conjunction with polyclonal antibodies raised against proteins and peptides of PSII and chl and/b LHC.